1. Field of the Invention
The present invention relates to a system for superimposing color characters or the like on nonstandard signals such as video reproduction signals.
2. Description of the Prior Art
For example, while reproducing a video tape, a remainder of the tape or other information may be displayed on a screen. This display is effected by superimposing a character signal on a video signal of NTSC, PAL or other system reproduced by a video deck. Hitherto, when combining the video signal being reproduced and a character signal desired to be added, a circuit as shown in FIG. 1 was used. The video signal is given to an analog switch 2, and a character signal is fed to an analog switch 4. Changeover signal is applied to both analog switches 2 and 4. When the changeover signal is at low level, the video signal is delivered, and when at high level, the character signal is delivered. Therefore, at the timing of insertion of character signal, by setting the changeover signal to high level, a superimposed video signal is obtained. For example, when a character signal is shown in FIG. 2B is added to a reproduction video signal as shown in FIG. 2A, a superimposed video signal as shown in FIG. 2C is obtained. When this video signal is displayed, a white character is superimposed and displayed in the video image. In FIG. 2A, meanwhile, numeral 6 denotes a horizontal synchronizing signal, and numeral 8 denotes a burst signal.
However, in such superimposing circuit, only the luminous component is contained in the added signal. Therefore, only white, black or gray characters can be superimposed.
Another problem in superimposing is that a video signal reproduced by VTR or a video signal of a household computer is a nonstandard signal having the frequency ratio of a burst signal to a horizontal sync. pulse deviated from the standard of NTSC, PAL and others. When superimposing on such nonstandard video signal, following problem occurs. When a display position of a superimposing character is determined by using the burst signal of nonstandard video signal as the reference clock, undesirable drift (jitter) is caused in the superimposed character.
Further, there is some case that only the added signal is displayed without the video signal. In this case, therefore, since the signal necessary for the sync. pulse or the like is not fed, the added signal cannot be displayed.